Add new rule 'exists'
Add new rule 'exists'

Rule is satisfied if the entry exists in the data field not taking into
account it's value at all:

Example:
{
    "utterance": "set an alarm on july 4th 2016 at 3pm",
    "evaluation_timeout": 10,
    "assert": "[['and', ['exists', 'ampm'], ['exists', 'time'] ]]"
}

Dark5ide/mycroft-core,linuxipho/mycroft-core,MycroftAI/mycroft-core,linuxipho/mycroft-core,Dark5ide/mycroft-core,forslund/mycroft-core,MycroftAI/mycroft-core,forslund/mycroft-core